A thermal treatment process to increase the conductivity of pyrolytic manganese dioxide coatings for valve metal nitride electrolytic capacitors.
Manganese dioxide coatings on both internal and external surfaces of anodized electrolytic capacitor anodes serve as the cathode for most commercially available xe2x80x9csolidxe2x80x9d capacitors. Typically, powdered tantalum or other suitable valve metal is pressed and sintered to form a porous anode body which serves as the anode. Other suitable valve metals include titanium, niobium, zirconium, aluminum, hafnium, tungsten, or mixtures, alloys, or metallic-glass compositions of these metals. A capacitor dielectric is then formed by electrolytic oxidation of the surfaces of the porous anode body. A semiconductor manganese dioxide coating is then deposited upon these surfaces to serve as the cathode in the electrolytic capacitor construction.
The manganese dioxide coating is typically produced by dipping the anodized anodes in an aqueous manganous nitrate solution followed by pyrolytic decomposition of the manganous nitrate to manganese dioxide in an oven, usually at a temperature of 130-270xc2x0 C., for sufficient time to substantially complete the pyrolytic conversion of the manganous nitrate. This process is repeated several times using various manganous nitrate concentrations to form a semi-conducting film providing adequate coverage of all anode surfaces.
Modern electronics circuits employing capacitors function most efficiently with capacitors having relatively low equivalent series resistance (ESR) and dissipation factor (df). In order to produce electrolytic capacitors containing manganese dioxide cathodes and having relatively low ESR and df, it is important that the capacitors contain highly conductive manganese dioxide.
Manganese dioxide, both naturally occurring and synthetic, is a complex substance which occurs in a variety of crystal forms, densities, hydration states, and metal/oxygen stoichiometries. These differences impact the electrical conductivity of manganese dioxide to a marked degree. Generally speaking, the most conductive material appears to be the beta crystal form, which also has the highest density and lowest water content, and is closest to true MnO2 stoichiometry.
Capacitor manufacturers have attempted to maximize the electrical conductivity of pyrolytic manganese dioxide by optimizing the manganous nitrate specific gravity dip sequence, oven temperature, and the composition of the oven atmosphere. A process for injecting steam into a conversion oven was described in U.S. Pat. Nos. 3,337,429. 5,622,746 describes a process for producing high conductivity pyrolytic manganese dioxide coatings by injecting highly oxidizing species, such as nitric acid or ozone, into the pyrolysis oven. However with nitric acid injection, control of the atmosphere is difficult, pyrolysis oven redesign is necessary to limit the volumes of nitric acid required, and the acidic atmosphere tends to cause the manganous nitrate to splatter, depositing MnO2 on the capacitor positive lead. It is also desirable from an environmental standpoint to avoid the generation of excess NO2 gases which result from the decomposition of the additional nitric acid injected into the pyrolysis oven.
U.S. Pat. Nos. 4,038,159 and 4,042,420 disclose highly conductive manganese dioxide coatings on tantalum capacitors produced through the thermal decomposition of aqueous manganous nitrate solutions in a small, positive pressure, radiant energy oven. However, there is still the problem of manganous nitrate splatter with small, positive pressure radiant furnaces. See, for example, U.S. Pat. No. 4,038,159, column 5, lines 44-52.
U.S. Pat. No. 3,801,479 describes a method for incorporating electrolytic manganese dioxide into a tantalum capacitor anode followed by a heat treatment or high temperature anodizing step to improve the leakage current of capacitors via restoration of stoichiometry of the tantalum oxide dielectric with oxygen from water released by the electrolytic manganese dioxide (FIG. 1 and equation 7, line 14, column 5).
The literature of the tantalum capacitor industry generally describes the product of the pyrolytic decomposition of manganous nitrate as beta-MnO2. Wiley, et. al. describes the pyrolytic conversion of manganous nitrate in test tubes to form manganese dioxide in the paper entitled xe2x80x9cThe Electrical Resistivity of Pyrolytic Beta MnO2,xe2x80x9d Journal of the Electrochemical Society, Vol. 111, June 1964. The authors indicate that x-ray diffraction patterns of the samples produced were consistent with beta-MnO2. The resistivities of the samples produced were essentially constant for conversion temperatures between 150 and 370xc2x0 C.
In a paper entitled xe2x80x9cElectrical Properties of Manganese Dioxide and Manganese Sesquioxide,xe2x80x9d Journal of the Electrochemical Society, Vol. 117, No. 7, July 1970, Peter Klose describes manganese oxides produced by the pyrolytic decomposition of manganous nitrate in a variety of vessels. While Klose did not perform x-ray diffraction analysis of his samples, others in the literature routinely refer to them as beta-MnO2 (e.g. E. Preisler, xe2x80x9cSemiconductor Properties of Manganese Dioxide,xe2x80x9d Journal of Applied Electrochemistry, 6,311 (1976) and Jian-Bao Li, et. al., xe2x80x9cElectrical Properties of Beta and Gamma Type Manganese (IV) Oxides,xe2x80x9d Journal of the Ceramic Society of Japan, 96, 74 (1988)). U.S. Pat. No. 3,801,479 describes manganese dioxide used in solid tantalum capacitors as beta-MnO2.
In a paper entitled xe2x80x9cElectrical and Physical Properties of MnO2, Layer for the High Performance Tantalum Solid Electrolytic Capacitorxe2x80x9d, presented at the 2nd Manganese Dioxide Symposium in Tokyo in 1980, the researchers claim that the pyrolytic conversion of manganous nitrate in both forced convection and radiation furnaces result in beta-MnO2 over a wide range of conversion temperatures (200 to 300xc2x0 C.).
U.S. Pat. No. 3,801,479 describes the pyrolytic MnO2 on tantalum capacitors as beta-MnO2 (in FIG. 1 and in lines 46-49, column 3). This patent discloses the departure from stoichiometry produced in the anodic tantalum oxide by the pyrolysis process and exploits the resulting large dependence of the anodic oxide resistivity upon temperature to produce a uniform layer of electrolytic manganese dioxide on the pyrolized capacitor bodies from solutions containing manganese ions and maintained at a temperature between 50xc2x0 C. and 99xc2x0 C. (claim 7). The uniform currents required for the production of the electrolytic manganese dioxide layer are not obtained unless the anodic tantalum oxide stoichiometry is first disturbed by exposure to pyrolysis temperatures (lines 8-12, column 4). The leakage current of capacitors produced by the process described in U.S. Pat. No. 3,801,479 is reduced to produce high-quality capacitors by restoration of the anodic tantalum oxide stoichiometry via a heat treatment step during which the tantalum anode is externally biased neutral or positive (lines 31-33, column 5) to produce migration of oxygen to the tantalum oxide surface (equation 7, line 15, column 5). The importance of the application of voltage at temperatures above 110xc2x0 C. is stressed in lines 62-71, column 7.
While beta-MnO2 is formed by the pyrolytic decomposition of manganous nitrate in a test tube, unless special measures are taken to control the oven atmosphere, the primary product of the decomposition of manganous nitrate on tantalum capacitors is a form of MnO2 referred to as gamma-MnO2 or epsilon-MnO2(ahktenskite). X-ray diffraction studies of pyrolytic manganese dioxide sample produced under a wide range of pyrolysis temperatures (130-330xc2x0 C.) in various atmospheres (dry to 75% steam) indicate the less conductive forms of MnO2 are the principle reaction product. Beta-MnO2 is produced by the decomposition of manganous nitrate if nitric acid is injected into the oven atmosphere (U.S. Pat. No. 5,622,746). This process however is difficult to control, and it is desirable to avoid the introduction of concentrated nitric acid into a production environment.
E. Preisler (xe2x80x9cSemiconductor Properties of Manganese Oxide,xe2x80x9d Journal of Applied Electrochemistry, 6,311 (1976) describes a heat treatment which transforms the properties of elecrodeposited manganese oxides from gamma-MnO2 to beta-MnO2. R. Giovanoli (xe2x80x9cA Review of Structural Data of Electrolytical and Chemical MnO2,xe2x80x9d 2nd MnO2 Symposium in Tokyo (1980) reported that lattice transformations could be observed in gamma-MnO2 chemically prepared from Na4Mn14O27*9H2O by digestion in dilute nitric acid (CMD). Evidence of the lattice transformations were also observed in gamma-MnO2 prepared by electrolytic methods (EMD). Jian-Bao Li, et al. (xe2x80x9cElectrical Properties of Beta- and Gamma-Type Manganese (IV) Oxides,xe2x80x9d Journal of the Ceramic Society of Japan, 96, 74 (1988) describe the effect of various heat treatments on beta-MnO2 and gamma-MnO2. Gamma-MnO2 samples exhibited minimum resistivity following heat treatment in the temperature range 350-400xc2x0 C. Beta-MnO2 exhibited small increases in resistivity following exposures to temperatures in excess of 150xc2x0 C. Peter Klose (xe2x80x9cElectrical Properties of Manganese Dioxide and Manganese Sesquioxide,xe2x80x9d Journal Of The Electrochemical Society, 111,656 (1960) also reports an increase in the resistivity of pyrolytic MnO2 following exposure to temperatures in excess of 200xc2x0 C. P. Fau, et al. (xe2x80x9cElectrical Properties of Sputtered MnO2 Thin Films,xe2x80x9d Applied Surface Science, 78, 203 (1994) describes a decrease in the resistivity of sputtered thin films of MnO2, by air annealing at temperatures up to 450xc2x0 C.
It is the object of the present invention to provide a low cost process for increasing the conductivity of pyrolytic manganese dioxide coatings for electrolytic capacitors.
It is a further object of this invention to provide a low cost method for producing capacitors which have low ESR.
It is still further object of the present invention to transform pyrolytic manganese dioxide produced by conventional pyrolysis ovens to a more conductive state in a single process step which avoids the problems of oven redesign, pyrolysis oven atmosphere control, acid injection, and manganous nitrate splatter.
It is still further an object of the present invention to produce highly conductive manganese dioxide coatings without resorting to a high temperature (e.g., 350-450xc2x0 C.) pyrolysis process. Such processes tend to produce devices having elevated leakage current. Such processes also increase high energy related operating costs and render control of the oven temperature and temperature more difficult as capacitor anodes are introduced into and removed from the pyrolysis oven.
Consistent with these objectives, the invention relates to a process of preparing an electrolytic capacitor comprising heating a manganese dioxide coated porous anodized valve metal nitride anode to a temperature of about 325xc2x0 C. to about 450xc2x0 C. to produce a thermally treated anode.
The invention is further directed to a process of preparing an electrolytic capacitor comprising heating a manganese dioxide coated porous anodized valve metal nitride anode to a temperature of about 200xc2x0 C. to about 250xc2x0 C. for a time sufficient for the anodized valve metal nitride anode to reach thermal equilibrium, and then increasing the temperature to a temperature of about 325xc2x0 C. to about 450xc2x0 C. to produce a thermally treated anode. Preferably, thereafter, the oven temperature is reduced to 200-250xc2x0 C. to reduce thermal shock of the anode.
The present invention is further directed to a process of preparing an electrolytic capacitor comprising impregnating a porous anodized valve metal nitride anode with an aqueous manganous nitrate solution; heating to a first temperature sufficient to cause pyrolytic decomposition of the manganous nitrate; repeating the impregnating and the heating to provide a manganese dioxide coated porous anodized valve metal nitride anode; and heating the coated porous valve metal nitride anode to a second temperature of about 325xc2x0 C. to about 450xc2x0 C. to produce a thermally treated anode.